From the experimental side, during the last decade we have witnessed
remarkable advances in the
observation of neutron stars:
the discovery of extremely massive neutron stars [1]}, [2]};
qualitative improvements in X-ray radius measurements
[3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]};
and the famous LIGO/Virgo detection of gravitational waves (GWs)
originating from the NS-NS merger GW170817 [10]}.
Increasingly stringent constraints have been placed on the EOS of
NS matter.
An accurate measurement of a compact object using Shapiro
delay [11]}
yielded \( 2.14^{+0.1}_{-0.09} \,\, M_{\odot }\)  for the J0740+6620 pulsar.
It has been argued that a handful of compact stars may achieve masses greater
than the PSR J0740+6620.
The events denoted as GW190814 [12]} and GW190425 [13]}
suggest that the NS mass can be larger than
\( 2.5 \,\, M_{\odot }\) .
Recent data (including a reliable determination of the radius) about the
pulsar PSR J0030+0451 were published in Refs.
[14]}, [15]}, [16]}, [17]}, [18]}. Finally, very recently [19]}
the NICER and XMM-Newton Collaborations presented a determination of the
mass and radius of the massive pulsar PSR J0740+6620.
